A wide variety of mental and physical processes are known to be controlled or influenced by neural activity in the central and peripheral nervous systems. For example, the neural functions in some areas of the brain (e.g., the sensory or motor cortices) are organized according to physical or cognitive functions. Several other areas of the brain also appear to have distinct functions in most individuals. In the majority of people, for example, the areas of the occipital lobes relate to vision, the regions of the left inferior frontal lobes relate to language, and the regions of the cerebral cortex appear to be involved with conscious awareness, memory, and intellect. Because of the location-specific functional organization of the brain, in which neurons at discrete locations are statistically likely to control particular mental or physical functions in normal individuals, stimulating neurons at selected locations of the central nervous system can be used to effectuate changes in cognitive and/or motor functions throughout the body.
In several existing applications, neural functions are tested, treated or augmented by electrical or magnetic stimulation powered by a neural stimulator that has a plurality of electrodes and a power source coupled to the electrodes. The electrodes can be implanted into the patient at a target site for stimulating the desired portions of the brain. For example, one existing technique for masking pain in a patient is to apply an electrical stimulus to a target stimulation site of the brain. In other applications, stimulation of an appropriate target site in the brain has shown promise for treating damage to and disease and disorders of the brain, including damage from strokes and treatment of Alzheimer's disease, depression, obsessive-compulsive behavior, and other disorders. In the surgical context, tcMEPs (transcranial stimulated electrical motor evoked potentials) have proven their advantages over wake-up tests in determining the integrity of the cortical spinal track during spinal and neurosurgical procedures.
The brain can be stimulated in several known fashions. One type of treatment is referred to as transcranial electrical stimulation (TES), which involves placing an electrode on the exterior of the patient's scalp and delivering an electrical current to the brain through the scalp and the skull. TES, however, is not widely used because the delivery of the electrical stimulation through the scalp and the skull causes patients a great amount of pain and the electrical field is difficult to direct or focus accurately due to shunting of electrical energy through the scalp.
Another type of treatment is transcranial magnetic stimulation (TMS), which involves using a high-powered magnetic field adjacent the exterior of the scalp over an area of the cortex. TMS does not cause the painful side effects of TES. Unfortunately, TMS is not presently effective for treating many patients because the existing delivery systems are not practical for applying stimulation over an adequate period of time. TMS systems, for example, are relatively complex and require stimulation treatments to be performed by a healthcare professional in a hospital or physician's office. The efficacy of TMS in longer-term therapies may be limited because it is difficult to (a) accurately localize the region of stimulation in a reproducible manner, (b) hold the device in the correct position over the cranium for the requisite period, and (c) provide stimulation for extended periods of time.
Another device for stimulating a region of the brain is disclosed by King in U.S. Pat. No. 5,713,922, the entirety of which is incorporated herein by reference. King discloses a device for cortical surface stimulation having electrodes mounted on a paddle that is implanted under the skull of the patient. These electrodes are placed in contact with the surface of the cortex to create “paresthesia,” which is a vibrating or buzzing sensation. Implanting the paddle typically requires removal of a relatively large (e.g., thumbnail-sized or larger) window in the skull via a full craniotomy. Craniotomies are typically performed under general anesthesia and subject the patient to increased chances of infection due to the invasive nature of the procedure.
In view of the foregoing, there remains a need in the art for neurostimulation methods and electrodes that deliver electrical energy to the target site with minimized shunting and pain to the patient and that are implantable via less invasive and traumatic procedures.